Characterization of liver GSD IX γ2 pathophysiology in a novel Phkg2-/- mouse model

INTRODUCTION

Liver Glycogen Storage Disease IX is a rare metabolic disorder of glycogen metabolism caused by deficiency of the phosphorylase kinase enzyme (PhK). Variants in the PHKG2 gene, encoding the liver-specific catalytic γ2 subunit of PhK, are associated with a liver GSD IX subtype known as PHKG2 GSD IX or GSD IX γ2. There is emerging evidence that patients with GSD IX γ2 can develop severe and progressive liver disease, yet research regarding the disease has been minimal to date. Here we characterize the first mouse model of liver GSD IX γ2.

METHODS

A Phkg2-/- mouse model was generated via targeted removal of the Phkg2 gene. Knockout (Phkg2-/-, KO) and wild type (Phkg2+/+, WT) mice up to 3 months of age were compared for morphology, Phkg2 transcription, PhK enzyme activity, glycogen content, histology, serum liver markers, and urinary glucose tetrasaccharide Glcα1-6Glcα1-4Glcα1-4Glc (Glc4).

RESULTS

When compared to WT controls, KO mice demonstrated significantly decreased liver PhK enzyme activity, increased liver: body weight ratio, and increased glycogen in the liver, with no glycogen accumulation observed in the brain, quadricep, kidney, and heart. KO mice demonstrated elevated liver blood markers as well as elevated urine Glc4, a commonly used biomarker for glycogen storage disease. KO mice demonstrated features of liver structural damage. Hematoxylin & Eosin and Masson's Trichrome stained KO mice liver histology slides revealed characteristic GSD hepatocyte architectural changes and early liver fibrosis, as have been reported in liver GSD patients.

DISCUSSION

This study provides the first evidence of a mouse model that recapitulates the liver-specific pathology of patients with GSD IX γ2. The model will provide the first platform for further study of disease progression in GSD IX γ2 as well as for the evaluation of novel therapeutics.

Variability of disease spectrum in children with liver phosphorylase kinase deficiency caused by mutations in the PHKG2 gene

Liver phosphorylase b kinase (PhK) deficiency (glycogen storage disease type IX), one of the most common causes of glycogen storage disease, is caused by mutations in the PHKA2, PHKB, and PHKG2 genes. Presenting symptoms include hepatomegaly, ketotic hypoglycemia, and growth delay. Clinical severity varies widely. Autosomal recessive mutations in the PHKG2 gene, which cause about 10-15% of cases, have been associated with severe symptoms including increased risk of liver cirrhosis in childhood. We have summarized the molecular, biochemical, and clinical findings in five patients, age 5-16 years, diagnosed with liver PhK deficiency caused by PHKG2 gene mutations. We have identified five novel and two previously reported mutations in the PHKG2 gene in these five patients. Clinical severity was variable among these patients. Histopathological studies were performed for four of the patients on liver biopsy samples, all of which showed signs of fibrosis but not cirrhosis. One of the patients (aged 9 years) developed a liver adenoma which later resolved. All patients are currently doing well. Their clinical symptoms have improved with age and treatment. These cases add to the current knowledge of clinical variability in patients with PHKG2 mutations. Long term studies, involving follow-up of these patients into adulthood, are needed.

Common mutation in the PHKA2 gene with variable phenotype in patients with liver phosphorylase b kinase deficiency

We found that the missense mutation p.Pro1205Leu in the PHKA2 gene is a common cause of hepatic phosphorylase-kinase deficiency in Dutch patients, suggesting a founder-effect. Most patients presented with isolated growth delay and diarrhea, prior to the occurrence of hepatomegaly, delaying diagnosis. Tetraglucoside excretion correlated with disease severity and was used to follow compliance. The clinical presentation and therapeutic requirements in the same mutation carriers were variable, and PhK deficiency necessitated tube-feeding in some children.

Liver glycogen storage diseases due to phosphorylase system deficiencies: diagnosis thanks to non invasive blood enzymatic and molecular studies

Glycogen storage disease (GSD) due to a deficient hepatic phosphorylase system defines a genetically heterogeneous group of disorders that mainly manifests in children. We investigated 45 unrelated children in whom a liver GSD VI or IX was suspected on the basis of clinical symptoms including hepatomegaly, increased serum transaminases, postprandial lactatemia and/or mild fasting hypoglycemia. Liver phosphorylase and phosphorylase b kinase activities studied in peripheral blood cells allowed to suspect diagnosis in 37 cases but was uninformative in 5. Sequencing of liver phosphorylase genes was useful to establish an accurate diagnosis. Causative mutations were found either in the PYGL (11 patients), PHKA2 (26 patients), PHKG2 (three patients) or in the PHKB (three patients) genes. Eleven novel disease causative mutations, five missense (p.N188K, p.D228Y, p.P382L, p.R491H, p.L500R) and six truncating mutations (c.501_502ins361pb, c.528+2T>C, c.856-29_c.1518+614del, c.1620+1G>C, p.E703del and c.2313-1G>T) were identified in the PYGL gene. Seventeen novel disease causative mutations, ten missense (p.A42P, p.Q95R, p.G131D, p.G131V, p.Q134R, p.G187R, p.G300V, p.G300A, p.C326Y, p.W820G) and seven truncating (c.537+5G>A, p.G396DfsX28, p.Q404X, p.N653X, p.L855PfsX87, and two large deletions) were identified in the PHKA2 gene. Four novel truncating mutations (p.R168X, p.Q287X, p.I268PfsX12 and c.272-1G>C) were identified in the PHKG2 gene and three (c.573_577del, p.R364X, c.2427+3A>G) in the PHKB gene. Patients with PHKG2 mutations evolved towards cirrhosis. Molecular analysis of GSD VI or IX genes allows to confirm diagnosis suspected on the basis of enzymatic analysis and to establish diagnosis and avoid liver biopsy when enzymatic studies are not informative in blood cells.

In Silico characterization of phosphorylase kinase: evidence for an alternate intronic polyadenylation site in PHKG1

Phosphorylase kinase (PhK), the key enzyme that regulates glycogenolysis, has traditionally been thought to be expressed predominantly in muscle and liver. In this study, we show by two different database searches (Expressed Sequence Tag and UniGene) that PhK gene expression occurs in at least 28-36 different tissues, and that the genes encoding the alpha, beta, and gamma subunits of PhK undergo extensive transcriptional processing. In particular, we have identified exon 6 of PHKG1 as a 3' composite terminal exon due to the presence of a weak polyadenylation and cleavage site in intron 6. We have verified biochemically that transcriptional processing of PHKG1 does occur in vivo; mRNA corresponding to the alternate variant is expressed in skeletal muscle, brain, heart, and tongue. In silico translation of this mRNA yields a PhK gamma subunit that contains the first 181 residues of the protein, followed by an additional 21 amino acids. The implication of this alternate processing is discussed within the context of gamma catalysis and regulation.

Fatal infantile cardiac glycogenosis with phosphorylase kinase deficiency and a mutation in the gamma2-subunit of AMP-activated protein kinase

A 10-wk-old infant girl with severe hypertrophy of the septal and atrial walls by cardiac ultrasound, developed progressive ventricular wall thickening and died of aspiration pneumonia at 5 mo of age. Postmortem examination revealed ventricular hypertrophy and massive atrial wall thickening due to glycogen accumulation. A skeletal muscle biopsy showed increased free glycogen and decreased activity of phosphorylase b kinase (PHK). The report of a pathogenic mutation (R531Q) in the gene (PRKAG2) encoding the gamma2 subunit of AMP-activated protein kinase (AMPK) in three infants with congenital hypertrophic cardiomyopathy, glycogen storage, and "pseudo PHK deficiency" prompted us to screen this gene in our patient. We found a novel (R384T) heterozygous mutation in PRKAG2, affecting an arginine residue in the N-terminal AMP-binding domain. Like R531Q, this mutation reduces the binding of AMP and ATP to the isolated nucleotide-binding domains, and prevents activation of the heterotrimer by metabolic stress in intact cells. The mutation was not found in DNA from the patient's father, the only available parent, and is likely to have arisen de novo. Our studies confirm that mutations in PRKAG2 can cause fatal infantile cardiomyopathy, often associated with apparent PHK deficiency.

Identification of Alu-mediated, large deletion-spanning introns 19-26 in PHKA2 in a patient with X-linked liver glycogenosis (hepatic phosphorylase kinase deficiency)

X-linked liver glycogenosis (XLG) is one of the most common glycogen storage diseases. We present the first case of a large PHKA2 gene deletion from intron 19 to intron 26 in an XLG patient. An aberrant cDNA with skipping of exons 20-26 was detected. Alu element-mediated unequal homologous recombination between an Alu-Jo in intron 19 and another Alu-Sg in intron 26 appears to be responsible for this deletion.

Multiple voxel 1H MR spectroscopy of phosphorylase-b kinase deficient patients (GSD IXa) showing an accumulation of fat in the liver that resolves with aging

BACKGROUND/AIMS

Phosphorylase-b deficient patients suffer from glycogen storage disease (GSD IXa) leading to liver enlargement which usually resolves during puberty and adolescence. This pathology has not yet been documented by (1)H MR spectroscopy (MRS) investigation.

METHODS

MRS of eight GSD IXa patients was performed in this study to assess whether or not liver fat content is elevated in GSD IXa and decreases with aging. An improvement in our MRS method compared with previous liver fat MRS studies is that we measured a plane of liver voxels at once rather than a single MRS voxel, yielding a reliable determination of liver fat content.

RESULTS

Fat contents of 3.4-10% were observed in young GSD IXa patients, as compared with 0.5-0.9% in controls, these dropped to control levels in patients past age 40 (r = -0.82; P < 0.01).

CONCLUSIONS

Liver fat content is increased in glycogen storage disease (GSD IXa) and normalizes with ageing. Assessing liver fat levels in this population is a novel and interesting concept. This could potentially enhance the understanding of liver function in that 20% of the population who has increased liver fat.

Fatal congenital heart glycogenosis caused by a recurrent activating R531Q mutation in the gamma 2-subunit of AMP-activated protein kinase (PRKAG2), not by phosphorylase kinase deficiency

Fatal congenital nonlysosomal cardiac glycogenosis has been attributed to a subtype of phosphorylase kinase deficiency, but the underlying genes and mutations have not been identified. Analyzing four sporadic, unrelated patients, we found no mutations either in the eight genes encoding phosphorylase kinase subunits or in the two genes encoding the muscle and brain isoforms of glycogen phosphorylase. However, in three of five patients, we identified identical heterozygous R531Q missense mutations of the PRKAG2 gene, which encodes the gamma 2-subunit of AMP-activated protein kinase, a key regulator of energy balance. Biochemical characterization of the recombinant R531Q mutant protein showed >100-fold reduction of binding affinities for the regulatory nucleotides AMP and ATP but an enhanced basal activity and increased phosphorylation of the alpha -subunit. Other PRKAG2 missense mutations were previously identified in patients with autosomal dominant hypertrophic cardiomyopathy with Wolff-Parkinson-White syndrome, characterized by juvenile-to-adult clinical onset, moderate cardiac glycogenosis, disturbed excitation conduction, risk of sudden cardiac death in midlife, and molecular perturbations that are similar to--but less severe than--those observed for the R531Q mutation. Thus, recurrent heterozygous R531Q missense mutations in PRKAG2 give rise to a massive nonlysosomal cardiac glycogenosis of fetal symptomatic onset and rapidly fatal course, constituting a genotypically and clinically distinct variant of hypertrophic cardiomyopathy with Wolff-Parkinson-White syndrome. R531Q and other PRKAG2 mutations enhance the basal activity and alpha -subunit phosphorylation of AMP-activated protein kinase, explaining the dominant nature of PRKAG2 disease mutations. Since not all cases displayed PRKAG2 mutations, fatal congenital nonlysosomal cardiac glycogenosis seems to be genetically heterogeneous. However, the existence of a heart-specific primary phosphorylase kinase deficiency is questionable, because no phosphorylase kinase mutations were found.

Biochemical characteristics and increased tetraglucoside excretion in patients with phosphorylase kinase deficiency

Patients with glycogen storage disease type IXa present with infantile hepatomegaly and a specific growth pattern, and variable biochemical alterations in blood. We studied the clinical and biochemical characteristics including the urinary oligosaccharide excretion of seven unrelated children. The urinary tetraglucoside excretion was increased in four children, three of whom had persistently high cholesterol and triglyceride concentrations. We propose screening for urine tetraglucoside excretion and the measurement of serum cholesterol in patients with growth delay and/or hepatomegaly to assess a possible glycogenosis.

Myopathy and phosphorylase kinase deficiency caused by a mutation in thePHKA1 gene

Phosphorylase kinase (PhK) deficiency is the underlying cause of variable clinical symptoms depending on the tissues involved. Until today, only a few cases of myopathy associated with muscle PhK deficiency caused by a mutation in the gene encoding the alpha subunit of phosphorylase kinase (PHKA1) have been reported. We describe a male patient with myopathy and absent muscle PhK activity caused by a frameshift mutation in the gene encoding the alpha subunit of PhK on chromosome Xq12-q13.

Severe phenotype of phosphorylase kinase-deficient liver glycogenosis with mutations in the PHKG2 gene

Phosphorylase kinase-deficient liver glycogenosis manifests in infancy with hepatomegaly, growth retardation, and elevated plasma aminotransferases and lipids. It can be caused by mutations in three different genes of phosphorylase kinase subunits: PHKA2, PHKB, and PHKG2. It is usually a benign condition, often with complete resolution of symptoms during puberty. A minority of patients displays a more severe phenotype with symptomatic fasting hypoglycemia and abnormal liver histology that may progress to cirrhosis. Three patients with liver cirrhosis in childhood analyzed previously all had PHKG2 mutations. This suggested that this genotype may generally cause a more severe clinical manifestation, but to date PHKG2 mutations have been identified in only seven patients. Here, we report mutation analysis in three new patients with liver phosphorylase kinase deficiency and recurrent hypoglycemia, liver fibrosis, and lack of glucagon response but no overt cirrhosis. In all three patients, PHKG2 mutations were found (H89fs[insC], E157K, D215N, W300X). Three of these mutations are novel, bringing the total number of distinct human PHKG2 mutations to 11, found in 10 patients. We conclude that liver phosphorylase kinase deficiency with a severe phenotype, with or without cirrhosis, is indeed often caused by PHKG2 mutations. These patients require active measures to maintain normoglycemia (raw cornstarch, nocturnal tube feeding), which may also alleviate growth retardation and the development of abnormal liver histology.

Muscle glycogenosis with low phosphorylase kinase activity: mutations in PHKA1, PHKG1 or six other candidate genes explain only a minority of cases

Muscle-specific deficiency of phosphorylase kinase (Phk) causes glycogen storage disease, clinically manifesting in exercise intolerance with early fatiguability, pain, cramps and occasionally myoglobinuria. In two patients and in a mouse mutant with muscle Phk deficiency, mutations were previously found in the muscle isoform of the Phk alpha subunit, encoded by the X-chromosomal PHKA1 gene (MIM # 311870). No mutations have been identified in the muscle isoform of the Phk gamma subunit (PHKG1). In the present study, we determined Q1the structure of the PHKG1 gene and characterized its relationship to several pseudogenes. In six patients with adult- or juvenile-onset muscle glycogenosis and low Phk activity, we then searched for mutations in eight candidate genes. The coding sequences of all six genes that contribute to Phk in muscle were analysed: PHKA1, PHKB, PHKG1, CALM1, CALM2 and CALM3. We also analysed the genes of the muscle isoform of glycogen phosphorylase (PYGM), of a muscle-specific regulatory subunit of the AMP-dependent protein kinase (PRKAG3), and the promoter regions of PHKA1, PHKB and PHKG1. Only in one male patient did we find a PHKA1 missense mutation (D299V) that explains the enzyme deficiency. Two patients were heterozygous for single amino-acid replacements in PHKB that are of unclear significance (Q657K and Y770C). No sequence abnormalities were found in the other three patients. If these results can be generalized, only a fraction of cases with muscle glycogenosis and a biochemical diagnosis of low Phk activity are caused by coding, splice-site or promoter mutations in PHKA1, PHKG1 or other Phk subunit genes. Most patients with this diagnosis probably are affected either by elusive mutations of Phk subunit genes or by defects in other, unidentified genes.

[Glycogenesis due to adult phosphorylase kinase deficiency]

A 42-year-old man presented exercise-induced muscle pain without myogloburia since the age of 12 years. Histochemistry and electronmicroscopy of a muscle biopsy revealed subsarcolemmal and inter-myofibrillar accumulation of glycogen. Exercise on a bicycle ergometer produced a normal raise of lactate. Biochemical study showed a partial defect in phosphorylase activity.

Detection of PHKA2 gene mutation in four Japanese patients with hepatic phosphorylase kinase deficiency

We analyzed the PHKA2 gene in four Japanese families with hepatic phosphorylase kinase (PhK) deficiency. Mutational analysis of PHKA2 cDNA was performed by reverse-transcribed polymerase chain reaction (RT-PCR) and direct sequencing, and each mutation was confirmed on the genomic DNA. In boys with low erythrocyte PhK activity (i.e., x-linked liver glycogenosis [XLG] type I), deletion of exon 2 (splice site mutation of 79-1 G > T) or nonsense mutation of Q1169X or R497X was identified. However, missense mutation of R295C was identified in one boy with normal erythrocyte PhK activity (i.e., XLG type II). This mutation was not found in 100 control alleles, and was considered responsible for presentation of the XLG type II phenotype. Excluding Q1169X, all mutations detected in this study represented novel mutations. All mothers were found to be heterozygous carriers of the mutations. Gene analysis was confirmed to represent a useful procedure for diagnosing XLG type II, for which liver biopsy had previously been required to detect hepatic PhK deficiency.

Muscle biopsy in the evaluation of patients with modestly elevated creatine kinase levels

The utility of muscle biopsy in patients with modest elevations of serum creatine kinase (CK) level but normal neurological examinations and nondiagnostic electrodiagnostic studies is uncertain. We performed systematic, extensive studies on muscle biopsies of 20 such patients. A definitive diagnosis was arrived at in only 1 by histochemical studies, although 4 others demonstrated minor myopathic changes. Biochemical evaluation led to a diagnosis in an additional 5. Muscle biopsy is useful for evaluating such patients, but extensive studies of the muscle are necessary.

Characteristic growth pattern in male X-linked phosphorylase-b kinase deficiency (GSD IX)

Growth retardation is one of the clinical characteristics of glycogen storage disease (GSD) type IX. Initial growth retardation has been described in a few case reports, followed by a complete catch-up in growth. This study aimed to determine the growth pattern of patients with GSD IX. Growth charts of 51 male Dutch patients with GSD IX (age 0-33 years, median follow-up time 8.3 years (range 0-30.5 years)) were studied retrospectively and compared with Dutch standard growth charts. Patients had a normal height at birth, significant growth retardation between the ages of 2 and 10 years (mean z-score -1.96), delayed growth spurt in puberty and catch-up towards quite normal final height (mean z-score -0.55). We conclude that GSD IX patients have a specific growth pattern characterized by initial growth retardation, a late growth spurt and complete catch-up in final height. Intervention for growth retardation is therefore in general not warranted. It is speculated that mild hypoglycaemia related to the disorder may cause endocrine changes. Because the glucose need per kg bodyweight decreases with age, the enzyme defect becomes less important with ageing and the effect on growth diminishes.

Adult-onset exercise intolerance due to phosphorylase b kinase deficiency

Muscle-specific phosphorylase b kinase deficiency is an unusual form of glycogen storage disorder. The majority of patients are male with an age at diagnosis between 15 to 36 years. Clinical features include exercise intolerance, myalgia and muscle weakness. A forearm ischaemic exercise test is usually normal and histochemical staining for myophosphorylase positive. The demonstration of reduced muscle phosphorylase b kinase activity by biochemical assay confirms the diagnosis. We report a 36 year old male with phosphorylase b kinase deficiency and symptom onset in adult life.

Fetal-onset severe skeletal muscle glycogenosis associated with phosphorylase-b kinase deficiency

We report on a premature newborn girl delivered after 32 weeks of gestation by cesarean section after sparse limb movements, fetal tachycardia and late heart rate decelerations had suggested fetal distress. Following 1 day of mechanical ventilation, adequate pulmonary gas exchange was achieved by spontaneous breathing. Main symptoms were virtually complete absence of spontaneous movements, increased flexor tonus of the extremities, and hypotonia of the trunk. Inability to suck or swallow required nasogastric gavage feeding. There were no hypoglycemic episodes. Echocardiography revealed normal myocardial function. Creatine kinase was 237 U/I at 2 days of life, declining to normal values thereafter. Muscle biopsy revealed increased glycogen storage with subsarcolemmal glycogen deposits and low phosphorylase-a activity while total phosphorylase was normal after in vitro activation, suggestive of phosphorylase-b kinase deficiency. No mutation was detected in exon 1 of the myophosphorylase gene. No psychomotor development was observed, and the infant died of central apnea at 3 months of age.

Glycogen storage diseases. Phenotypic, genetic, and biochemical characteristics, and therapy

The glycogen storage diseases are caused by inherited deficiencies of enzymes that regulate the synthesis or degradation of glycogen. In the past decade, considerable progress has been made in identifying the precise genetic abnormalities that cause the specific impairments of enzyme function. Likewise, improved understanding of the pathophysiologic derangements resulting from individual enzyme defects has led to the development of effective nutritional therapies for each of these disorders. Meticulous adherence to dietary therapy prevents hypoglycemia, ameliorates the biochemical abnormalities, decreases the size of the liver, and results in normal or nearly normal physical growth and development. Nevertheless, serious long-term complications, including nephropathy that can cause renal failure and hepatic adenomata that can become malignant, are a major concern in GSD-I. In GSD-III, the risk for hypoglycemia diminishes with age, and the liver decreases in size during puberty. Cirrhosis develops in some adult patients, and progressive myopathy and cardiomyopathy occur in patients with absent GDE activity in muscle. It remains unclear whether these complications of glycogen storage disease can be prevented by dietary therapy. Glycogen storage diseases caused by lack of phosphorylase activity are milder disorders with a good prognosis. The liver decreases in size, and biochemical abnormalities disappear by puberty.

Infantile hypertrophic cardiomyopathy of glycogenosis type IX: isolated cardiac phosphorylase kinase deficiency

Glycogen storage disease confined to the heart due to cardiac phosphorylase kinase deficiency causes a fatal infantile cardiomyopathy. Cardiomegaly can be detected in utero and is progressive. Electrocardiographic and echocardiographic findings are characteristic but not specific; these include large QRS complexes, short PR interval, and a hypertrophic nonobstructive pattern. Conclusive diagnosis requires biochemical analysis of myocardium, which may not be possible premortem due to the amount of tissue required. Pathologic examination of a standard cardiac biopsy can provide a presumptive diagnosis. There is no current treatment except a heart transplant. Infants succumb to heart failure and/or respiratory compromise due to pulmonary compression. This is a rare entity; only three cases have been reported to our knowledge. We report two additional cases.

Mutational analyses in four Japanese families with X-linked liver phosphorylase kinase deficiency type 1

We analysed the gene of the human alpha-subunit of liver phosphorylase kinase (PHKA2) in four Japanese families with X-linked liver phosphorylase kinase deficiency type 1 by RT-PCR followed by PCR-single-strand conformation polymorphism and direct DNA sequencing. In this study, two novel mutations (Y116D and 2675A-->G) and one mutation previously reported (P1205L) were identified, revealing molecular heterogeneity in Japanese patients. Considering the dissimilarity in phenotype among our patients even with an identical mutation in the PHKA2 gene, it seems that each genetic deficiency in this gene may not be the only factor to determine the clinical heterogeneity in this disease.

Metabolic causes of recurrent rhabdomyolysis

OBJECTIVES

The aim of the study was to evaluate the biochemical causes of recurrent rhabdomyolysis in Finland.

MATERIAL AND METHODS

We examined 22 patients with recurrent rhabdomyolysis, and 26 patients with one episode of rhabdomyolysis or other symptoms compatible with metabolic myopathy. Muscle histopathology and activities of phosphorylase (PHRL) (total and active), phosphofructokinase (PFK), carnitine palmitoyltransferase (CPT) and myoadenylate deaminase (MAD) were studied. The limit of enzyme deficiency was defined as enzyme activity less than 5% of the mean of the control subjects.

RESULTS

We found 4 patients with muscle PHRL deficiency, 1 patient with PFK deficiency and 1 patient with evidence of phosphorylase kinase deficiency. One patient had Becker's muscle dystrophy, 2 patients had unspecified dystrophies, 1 patient had Miyoshi myopathy, and 1 patient had a form of mitochondrial encephalomyopathy (MELAS).

CONCLUSION

Enzyme defects were found in 23% of the patients with recurrent rhabdomyolysis. Other muscle diseases, muscular dystrophies or myopathies, were detected in 18% of these patients, emphasizing the value of clinical and histopathological examination of patients with previous rhabdomyolysis.

A splice junction mutation in the alpha(M) gene of phosphorylase kinase in a patient with myopathy

In a 28-year-old man with myopathy and phosphorylase kinase (PhK) deficiency, we found a G-to-C substitution at the 5' end of an intron in the muscle-specific alpha-subunit gene. The mutation destroys the high-consensus GT sequences at the 5' splice junction of the intron, which causes skipping of the preceding exon. This is the second molecular genetic defect identified in the myopathic variant of PhK deficiency.

Variability of biochemical and clinical phenotype in X-linked liver glycogenosis with mutations in the phosphorylase kinase PHKA2 gene

X-linked liver glycogenosis (XLG) resulting from phosphorylase kinase (Phk) deficiency is one of the most common forms of glycogen storage disease. It is caused by mutations in the gene encoding the liver isoform of the Phk alpha subunit (PHKA2). In the present study, we address the issue of phenotypic and allelic heterogeneity in XLG. We have identified mutations in seven male patients. One of these patients represents the variant biochemical phenotype, XLG subtype 2 (XLG2), where Phk activity is low in liver but normal or even elevated in erythrocytes. He carries a K189E missense mutation, which adds to the emerging evidence that XLG2 is associated with missense mutations clustering at a few sites. Two patients display clinical phenotypes unusual for liver Phk deficiency, with dysfunction of the kidneys (proximal renal tubular acidosis) or of the nervous system (seizures, delayed cognitive and speech abilities, peripheral sensory neuropathy), respectively, in addition to liver glycogenosis. In the patient with kidney involvement, we have identified a missense mutation (P399S) and a trinucleotide deletion (2858del3) leading to the replacement of two amino acids by one new residue (N953/L954I), and a missense mutation has also been found in the patient with neurological symptoms (G1207W). These two cases demonstrate that PHKA2 mutations can also be associated with uncommon clinical phenotypes. Finally, in four typical XLG cases, we have identified three truncating mutations (70insT, R352X, 567del22) and an in-frame deletion of eight well-conserved amino acids (2452del24). Together, this study adds eight new mutations to the previously known complement of sixteen PHKA2 mutations. All known PHKA2 mutations but one are distinct, indicating pronounced allelic heterogeneity of X-linked liver glycogenosis with mutations in the PHKA2 gene.

Infantile muscle phosphorylase-b-kinase deficiency. A case report

A Turkish girl is described who showed a severe floppy infant syndrome and respiratory failure at birth. She suffered upper respiratory tract infections and pneumonia. She was ventilated and had hypercapnoea secondary to bradypnoea. Biochemical analysis of skeletal muscle revealed a slightly increased glycogen content, and enzymatic analysis revealed a muscle phosphorylase-b-kinase deficiency. The infant succumbed after 140 days due to persistent apnoea and asystole. Isolated muscle phosphorylase-b-kinase deficiency should be considered as a possible diagnosis in floppy infants.

Liver glycogenosis due to phosphorylase kinase deficiency: PHKG2 gene structure and mutations associated with cirrhosis

Mutations in three different genes of phosphorylase kinase (Phk) subunits, PHKA2, PHKB and PHKG2, can give rise to glycogen storage disease of the liver. The autosomal-recessive, liver-specific variant of Phk deficiency is caused by mutations in the gene encoding the testis/liver isoform of the catalytic gamma subunit, PHKG2. To facilitate mutation detection and to improve our understanding of the molecular evolution of Phk subunit isoforms, we have determined the structure of the human PHKG2 gene. The gene extends over 9.5 kilonucleotides and is divided into 10 exons; positions of introns are highly conserved between PHKG2 and the gene of the muscle isoform of the gamma subunit, PHKG1. The beginning of intron 2 harbors a highly informative GGT/GT microsatellite repeat, the first polymorphic marker in the PHKG2 gene at human chromosome 16p11.2-p12.1. Employing the gene sequence, we have identified homozygous translation-terminating mutations, 277delC and Arg44ter, in the two published cases of liver Phk deficiency who developed cirrhosis in childhood. As liver Phk deficiency is generally a benign condition and progression to cirrhosis is very rare, this finding suggests that PHKG2 mutations are associated with an increased cirrhosis risk.

Autosomal recessive phosphorylase kinase deficiency in liver, caused by mutations in the gene encoding the beta subunit (PHKB)

The association of autosomal recessive phosphorylase kinase deficiency in liver of a 3 1/2-year-old female child with mutations in the gene encoding the common part of the beta subunit of phosphorylase kinase is reported. The proband had a severe deficiency of phosphorylase kinase in liver, while the phosphorylase kinase activity in erythrocytes was only slightly diminished. She had no symptoms of muscle involvement. The complete coding sequences of the liver gamma subunit and of the beta subunit of phosphorylase kinase of the proband were analyzed for the presence of mutations, by either reverse-transcribed PCR or SSCP analysis. Three deviations from the normal sequence were found in the region encoding the common part of the beta subunit of phosphorylase kinase-namely, a 1827G-->A (W609X) transition, a 2309A-->G (Y770C) transition, and a deletion of nucleotides 2896-2911-whereas no mutations were detected in the sequence encoding the liver gamma subunit of phosphorylase kinase. The 1827G-->A mutation and the deletion both result in the formation of early stop codons. Investigation of DNA showed that the deletion is caused by a splice-acceptor site mutation (IVS30(-1),g-->t). Family analysis revealed that the 1827G-->A and IVS30(-1),g-->t substitutions are located on different parental chromosomes and that compound heterozygosity for these mutations segregates with the disease. The 2309A-->G mutation was detected in 2%-3% of the normal population. Thus, it is concluded that the deficiency of phosphorylase kinase in this proband is caused by compound heterozygosity for the 1827G-->A and the IVS30(-1),g-->t mutations and that the 2309A-->G mutation is a polymorphism. This implies that a defect in the sequence encoding the common part of the beta subunit of phosphorylase kinase may present as liver phosphorylase kinase deficiency.

Autosomal recessive liver phosphorylase kinase deficiency caused by a novel splice-site mutation in the gene encoding the liver gamma subunit (PHKG2)

To facilitate mutation analysis of patients with an autosomal recessive form of liver phosphorylase kinase deficiency, the genomic structure of the gene encoding the testis/liver gamma subunit (PHKG2) was established. The gene consist of 10 exons. The translation start site is located in exon 2. Analysis of DNA from two female siblings, affected with liver phosphorylase kinase deficiency, by exon specific amplification followed by direct sequencing, revealed a single donor splice site mutation in the PHKG2 gene, IVS4 + 1(g --> a). The mutation leads to the skipping of exon 4, which results in a frameshift, starting at nucleotide 272, a premature stop codon after 32 additional amino acids, and subsequent loss of the catalytic site. It is concluded that deficiency of phosphorylase kinase in liver of the patients is caused by the IVS4 + 1(g --> a) mutation. In the patients described here, this genotype is associated with development of liver fibrosis.

Autosomal glycogenosis of liver and muscle due to phosphorylase kinase deficiency is caused by mutations in the phosphorylase kinase beta subunit (PHKB)

Glycogen storage disease due to phosphorylase kinase deficiency occurs in several variants that differ in mode of inheritance and tissue-specificity. This heterogeneity is suspected to be largely due to mutations affecting different subunits and isoforms of phosphorylase kinase. The gene of the ubiquitously expressed beta subunit, PHKB, was a candidate for involvement in autosomally transmitted phosphorylase kinase deficiency of liver and muscle. To identify such mutations, the complete PHKB coding sequence was amplified by RT-PCR of RNA isolated from blood samples of patients and analyzed by direct sequencing of PCR products. The characterization of mutations was complemented by PCR of genomic DNA. In one female and four male patients, we identified five independent nonsense mutations (Y418ter; R428ter; Y974H+E975ter; Q656ter in two cases), one single-base insertion in codon N421, one splice-site mutation affecting exon 31, and a large deletion involving the loss of exon 8. Although these severe translation-disrupting mutations occur in constitutively expressed sequences of the only known beta subunit gene of phosphorylase kinase, PHKB, they are associated with a surprisingly mild clinical phenotype, affecting virtually only the liver, and relatively high residual enzyme activity of approximately 10%.

Mutations in the testis/liver isoform of the phosphorylase kinase gamma subunit (PHKG2) cause autosomal liver glycogenosis in the gsd rat and in humans

Heritable deficiency of phosphorylase kinase (Phk), a regulatory enzyme of glycogen metabolism, is responsible for 25% of all cases of glycogen storage disease and occurs with a frequency of -1 in 100,000 births. It is genetically and clinically heterogeneous, occurring in X-linked and autosomal-recessive forms and exhibiting various patterns of principally affected tissues (liver only, muscle only, liver and muscle, liver and kidney, heart only). This heterogeneity is thought to reflect the enzyme's structural complexity [subunit composition, (alpha beta gamma delta)4] and isoform diversity. Two isoforms encoded by separate genes are known for the subunits alpha (muscle [alpha M] and liver [alpha L isoforms) and gamma (muscle [gamma M] and testis [gamma T] isoforms), whereas only one gene appears to exist for the subunit beta. The subunit delta is calmodulin; identical calmodulins are expressed from three different human genes. Additional isoform diversity arises by differential mRNA splicing of the alpha M, alpha L and beta subunits. Mutations responsible for the various forms of Phk deficiency are sought in those subunit/isoform genes with a matching chromosomal location and tissue-specificity of expression. We report here that autosomal liver-specific Phk deficiency is associated with mutations in the gene encoding the testis/liver isoform of the catalytic gamma subunit (PHKG2). We found homozygous PHKG2 mutations in three human patients of consanguineous parentage and in the gsd (glycogen storage disease) rat strain, which is thus identified as an animal model for the human disorder. One human mutation is a single base-pair insertion in codon 89 that causes a frameshift and premature chain termination. The three other mutations result in non-conservative replacements of amino acid residues (V106E, G189E, D215N) that are highly conserved within the catalytic core regions of all protein kinases. These are the first mutations to be reported for an autosomal form of Phk deficiency. The findings suggest that the PHKG2 gene product is the predominant isoform of the catalytic gamma subunit of Phk not only in testis but also in liver, erythrocytes and, possibly, other non-muscle tissues.

[Exercise intolerance caused by muscular phosphorylase kinase deficiency. Contribution of in vivo metabolic studies]

A 33 year old man has been presenting since childhood an exertional muscle pain syndrome without myoglobinuria. Muscle biopsy revealed a vacuolar myopathy with glycogen excess in subsarcolemmal and intermyofibrillar spaces which was confirmed by electron microscopy. Plasma production of ammonia was abnormally high during exercise on a bicycle ergometer while the raise of lactate was normal. NMR spectroscopy showed an increased muscle glycogen content, with a slight and delayed drop of the pH during exercise. Phosphorylase b kinase activity was undetectable in muscle specimen whereas activities of others enzymes of carbohydrate metabolism were normal. Clinical presentation of our patient is compared to that of the reported cases of phosphorylase b kinase deficiency.

Genetic deficiencies of the glycogen phosphorylase system

Several types of glycogen storage disease attributable to a deficiency of phosphorylase or phosphorylase kinase have been described. These diseases have been divided according to clinical symptoms, mode of inheritance, and affected tissue. However, this classification is questionable, as the clinical symptoms of these different diseases are similar, the mode of inheritance is often difficult to establish, and the biochemical assays are subject to several technical problems. A better classification would be based upon the identification of mutations in the respective disease genes. The molecular heterogeneity, however, is large, and at least 10 genes are involved. Mutations have been found in the muscle phosphorylase gene in patients with muscle phosphorylase deficiency, in the gene encoding the liver alpha subunit of phosphorylase kinase in patients with X-linked liver glycogenosis, and in the gene for the muscle alpha subunit of phosphorylase kinase in a patient with muscle phosphorylase kinase deficiency. We review here the different deficiencies of the phosphorylase system.

Mutation hotspots in the PHKA2 gene in X-linked liver glycogenosis due to phosphorylase kinase deficiency with atypical activity in blood cells (XLG2)

In five cases of X-linked liver glycogenosis subtype 2 (XLG2), we have identified mutations in the gene encoding the liver isoform of the phosphorylase kinase alpha subunit (PHKA2). XLG2 is a rare variant of X-linked phosphorylase kinase (Phk) deficiency of the liver. Whereas in the more common form of X-linked hepatic Phk deficiency, XLG1, the enzyme's activity is decreased both in liver and in blood cells, Phk activity in XLG2 is low in liver but normal or even enhanced in blood cells. Although missense, nonsense and splicesite mutations in the PHKA2 gene were recently identified in several cases of XLG1, no mutations have yet been described for XLG2 and a molecular explanation for the peculiar biochemical phenotype of XLG2 has been lacking. All mutations found in the present study result in non-conservative amino acid replacements of residues that are absolutely conserved between the alpha L, alpha M and beta subunits of Phk [H132P, H132Y, R186H (twice) and D299G]. Strikingly, in two pairs of cases the mutations affect the same codon. These results demonstrate that: (i) XLG2 is caused by mutations in PHKA2 and is therefore allelic with XLG1; and (ii) XLG2 mutations appear to cluster in limited sequence regions or even individual codons.

Phosphorylase b kinase deficiency glycogenosis with cirrhosis of the liver

We describe an Arab girl with complete absence of phosphorylase b kinase activity in the liver, symptomatic hypoglycemia, and persistently elevated serum aminotransferase values whose symptoms did not lessen with age; sequential liver biopsies showed progression to cirrhosis. Cirrhosis could not be ascribed to any other known cause. We conclude that type IX glycogenosis is not always associated with a benign outcome.

Isolation of cDNA encoding the human liver phosphorylase kinase alpha subunit (PHKA2) and identification of a missense mutation of the PHKA2 gene in a family with liver phosphorylase kinase deficiency

X-linked liver glycogenosis (XLG) due to liver phosphorylase kinase (PHK) deficiency is the most frequent liver glycogen storage disease. The affected patients present in early childhood with hepatomegaly and growth retardation. We isolated and determined the structure of human liver alpha subunit of PHK (PHKA2) cDNA. The 3705 base pair open reading frame encodes a polypeptide of 1235 amino acid residues, and the deduced amino acid sequence shows 93 and 68% homology to that of rabbit liver alpha subunit of PHK and human muscle alpha subunit of PHK, respectively. We identified a missense mutation, a valine substitution for glycine at amino acid 193, in the PHKA2 gene of a family with XLG.

Dinucleotide repeat polymorphism within the PHKA1 gene at Xq12-q13

A polymorphic complex repeat including two (TG)n stretches was identified in the intron following codon 26 of the human gene encoding the muscle isoform of the phosphorylase kinase alpha subunit (PHKA1). It should be a useful marker for linkage analysis of families with heritable phosphorylase kinase deficiency and for gene mapping in the vicinity of the X inactivation center.

X-linked liver phosphorylase kinase deficiency is associated with mutations in the human liver phosphorylase kinase alpha subunit

Two Dutch patients with liver phosphorylase kinase (PhK) deficiency were studied for abnormalities in the PhK liver alpha (alpha L) subunit mRNA by reversed-transcribed-PCR (RT-PCR) and RNase protection assays. One patient, belonging to a large Dutch family that expresses X-linked liver PhK deficiency, had a C3614T mutation in the PhK alpha L coding sequence. The C3614T mutation leads to replacement of proline 1205 with leucine, which changes the composition of an amino acid region, containing amino acids 1195-1214 of the PhK alpha L subunit, that is highly conserved in different species. The patient showed normal levels of PhK alpha L mRNA. The second patient, from an unrelated family, was found to have a TCT (bp 419-421) deletion in the PhK alpha L coding sequence, resulting in a phenylalanine 141 deletion. The same deletion was found in the PhK alpha L coding sequence from lymphocytes of the patient's mother, together with a normal PhK alpha L coding sequence. The phenylalanine that is absent in the PhK alpha L coding sequence of the second patient is a highly conserved amino acid between species. Both the C3614T mutation and the TCT (bp 419-421) deletion were not found in a panel of 80 control X chromosomes. On the basis of these results, it is postulated that the mutations found are responsible for liver PhK deficiency in the two patients investigated.

[Five cases of phosphorylase b kinase deficiency affecting muscle or liver: clinical symptoms and diagnosis]

Muscle, erythrocyte and/or leukocyte phosphorylase b kinase (PBK) activities were measured in 5 patients and in 2 families. The relation between phenotype and enzyme deficiency in clinically variable samples based on measurement of erythrocytes, leukocytes or muscle was described. Three patients showed the enzyme deficiency in liver and the disease was transmitted as an X-linked recessive trait. In these cases, our results suggested that carrier detection was more reliable with erythrocytes than leukocytes. Two patients showed enzyme deficiency in muscle, and the mode of inheritance was not clear. In these cases, muscle biopsies were necessary for diagnosis.

Human muscle glycogenosis due to phosphorylase kinase deficiency associated with a nonsense mutation in the muscle isoform of the alpha subunit

Heritable phosphorylase kinase (Phk) deficiency is responsible for several forms of glycogen storage disease in humans and animals that differ in mode of inheritance and tissue-specificity. Mutations affecting different subunits and isoforms of Phk are expected to contribute to this heterogeneity. In the present study, we have investigated a case of muscle-specific, adult-onset Phk deficiency. The coding sequences of three candidate genes were analyzed by RT-PCR and sequencing: the muscle isoform of the alpha subunit (alpha M), a muscle-specifically expressed exon of the beta subunit, and the muscle isoform of the gamma subunit. Whereas the latter two sequences were found to be normal, we identified a nonsense mutation in alpha M. The condition of this patient therefore is a human homolog of the X-linked muscle Phk deficiency of I-strain mice. To our knowledge, this is the first description of a human Phk deficiency mutation.

Uncooked cornstarch treatment for hepatic phosphorylase kinase deficiency

A 5-year-old boy with short stature, hepatomegaly and motor weakness due to hepatic phosphorylase kinase deficiency is described. Laboratory data showed mild hypoglycaemia and metabolic acidosis, hepatic dysfunction, and a low insulin-like growth factor-I level. Mild hypoglycaemia, marked ketosis and insufficient growth hormone secretion were revealed at night. Serum total and free carnitine levels were low and the acyl/total carnitine ratio was high. Urinary acylcarnitine profile using fast atom bombardment and tandem mass spectrometry showed increased excretion of acetylcarnitine and dicarboxylylcarnitines. These endocrinological and metabolic abnormalities and clinical symptoms were improved with uncooked cornstarch treatment.

CONCLUSION

Uncooked cornstarch treatment may be helpful in hepatic phosphorylase deficiency.